When Moshe Rabbeinu hit the rock instead of speaking to it, he was punished for his error. "Because you did not believe in Me to cause My Name to be sanctified before the eyes of Bnei Yisroel, therefore, you will not lead this nation into the land that I have given them" (Bamidbar 20, 12). Rashi explains that had Moshe spoken to the rock, it would have made for a powerful lesson. "If a rock which cannot speak or hear, nor does it require sustenance, nevertheless heeds the word of Hashem, how much more so is it imperative that I [who speak, hear and need sustenance] heed the word of Hashem."
Rav Wolbe asks why it was crucial that Bnei Yisroel behold this specific miracle; didn't they live within the ananei hakavod and weren't they sustained by mann that fell from the heavens? They lived a life of constant miracles, so what would have been unique about the miracle of the rock that would have stimulated a Kiddush Hashem?
He answers with an important concept. Many people are completely out of touch with what is going on deep inside their souls i.e. their penimius. Much, if not all, of their knowledge has no effect on their internal world. They might know something to be true and even preach it to the masses, and nevertheless, fail to truly internalize it to the point that it makes a difference in their lives. The language that best conveys these messages to the recesses of our hearts and minds is hispa'alus - being intensely stimulated. Hispa'alus is a penetrating realization of a truth that plows through the many layers of our subconscious and strikes our internal chords. Therefore, it is a catalyst for us to change the way we have been doing things prior to this realization.
The chiddush that is attributed to Rav Yisroel Salanter is not that one must study Mussar; that was already a given for many generations. He introduced the concept of studying mussar b'hispa'alus - in a way that he who is studying is stimulated by the truth of what he learns. Mussar study in such a manner penetrates more deeply than the mere knowledge of the material being studied. It was for this reason that the miracle of the rock was so crucial. It would have forced them into a realization that affected them internally. A rock that heeds the word of Hashem obligates a human to act, at the very least, in a similar manner.
Chazal tell us, "One who sees the punishment of a sotah should become a nazir and abstain from wine." Instead of lambasting the society that bred such lewdness, one should redirect his criticism toward himself. He should acknowledge that he too has a yetzer hara that burns inside of him, and therefore, he must take the proper precautions lest he himself be seduced into sinning.
In truth, everything that we see or hear should be integrated into our lives in a way that is conducive to avodas Hashem. In a practical sense, this is best achieved when one studies mussar b'hispa'alus.